A cooling device is known from, for example, U.S. Pat. No. 5,535,593 A.
In electrical devices or machines comprising superconductors, such as for example motors, generators or superconducting current limiters, the superconductor has to be cooled and to this end is generally located in a cryostat which contains a cryogenic coolant, such as for example liquid neon or liquid nitrogen. In this case, a cooling device serves for recondensing evaporated coolant present in the cryostat. The cooling device, frequently also denoted as a refrigerator, generally comprises a closed circuit in which a working medium, for example helium gas, is compressed in a compressor and expanded again in a cooling unit and, as a result, discharges cooling power to the coolant located in the cryostat. The cooling device may, for example, operate according to the Gifford McMahon principle, according to the pulse tube principle or according to the Stirling principle.
Due to their high power density, small space requirement and other specific properties of the superconductor, electrical devices or machines comprising superconductors are eminently suitable for use in mobile devices, such as for example in ships or offshore platforms. Thus DE 10 2004 023 481 A1 and WO 03/047961 A2 disclose marine propulsion machines and generators comprising a rotor with a rotating high-temperature superconductor field winding, which is arranged in a cryostat in which neon is located at a temperature of 25 K as coolant for the superconductor. The cryostat is connected via a cryo-heat pipe to a cold head of a cooling device to which a compressor also belongs.
A short-circuit current protection system for ships and offshore installations comprising a superconducting current limiter is disclosed in EP 1 526 625 A1, in which the superconductor is arranged in a cryostat, in which liquid nitrogen is located at a temperature of 77 K as coolant for the superconductor. A cooling device serves for recondensing evaporated coolant, said cooling device comprising a cold head protruding into the cryostat and a compressor. The cooling device itself is not able to be regulated, but the regulation takes place indirectly by a reheating device which is attached to the cold head. The reheating device is switched on and off by a temperature regulating device, so that the temperature of the liquid nitrogen at 77 K is at ambient pressure. Due to its low maintenance requirement, an oil-free linear compressor is preferably used as the compressor.
For the use of electrical devices or machines comprising superconductors in mobile devices, in particular on ships or offshore platforms, care has to be taken that the operation of the cooling device is also able to be ensured in an inclined position of the components. Thus, for example, for use on ships, operation also has to be ensured at an inclined position of 22.5 degrees.
Compressors operating according to the reciprocating piston principle or helical compressors, are not suitable in this case, as they are lubricated by oil and therefore are not able to be inclined in operation. Oil-free linear compressors are, however, suitable. Such a linear compressor generally comprises two pistons of which at least one, preferably both synchronously relative to one another, is and/or are able to be moved by a linear motor at a frequency and a stroke in a linear manner relative to the respective other piston.
It is known in this case to control the power of such a compressor manually or automatically by varying the motor voltage and the piston frequency. As has been proven, however, such a control method is not suitable for ships as, for example, it does not take into account dependencies of the resonance frequency of the pistons on the filling pressure in the circuit and the temperature of the working medium. Moreover, an inclination or oblique position of the compressor also leads to a shifting of the operating point of the compressor. This has the result, firstly, that a defined cooling power is not able to be set. Secondly, this has the result that operating points are set at which the cooling device operates at a very poor level of efficiency and has a relatively high requirement for electrical energy. Shifting the operating point may also result in the risk of the pistons striking a housing of the compressor and thus to safety cut-outs of the compressor.